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A little spiritual guidance this time

Discussion in 'Fisher' started by Masonavenut, Aug 23, 2017.

  1. Masonavenut

    Masonavenut AK Subscriber Subscriber

    Messages:
    146
    Location:
    Winnetka, Ca.
    Yes Mr. Cat’ I’m very lucky to have a few really good friends.
    Larry, Tom has an idea for the antennae (hmm English spelling came out of my fingers). I wasn’t to concerned with it myself because I’ll prolly never use it but it’ll be nice to have it working. Of course now that I think of it there is a pretty cool oldies (50s/60s) station out here on A M.
    My cables and DAC showed up yesterday. My foot is tapping.
     

     

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  2. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
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    The 800c is telling you things subconciously. It want's you to listen to that oldies station on AM. AM is the new FM (want some of what I'm smoking???). :confused::eek: Plus 50's-60's stuff always sounds better on AM.
     
  3. Masonavenut

    Masonavenut AK Subscriber Subscriber

    Messages:
    146
    Location:
    Winnetka, Ca.
    Heh heh, the 800C seems to be a powerful force of nature. It found me out of a barn and now is exerting it’s will.:bowdown:
    Agree totally about 50s/60s music. That’s the way we all heard it.
     
  4. Masonavenut

    Masonavenut AK Subscriber Subscriber

    Messages:
    146
    Location:
    Winnetka, Ca.
    Ok, got her back today from Mr. Ishimoto. I’ll post pics but first a question/moan.
    Tom told me when I picked it up that I should not use a speaker switcher as the Fisher has reverse ground (IIRC) and will not properly ground through a selector box.
    Will you gents please comment on this as I have no idea. I always though ground was ground.
    The way I have my system set up is I am using a dual input box so that I can switch between receivers (at least that was the idea). I have one set of speakers hooked up.
    https://www.amazon.com/TC-7220-Ampl...d=1527187645&sr=8-1&keywords=tc+7220+switcher
    Do I need to re think my system or (the more fun fix) buy another set of speakers for the tube receivers.
    What do you guys do?

    BTW Larry, he really liked the Hayseed parts. I guess he was not familiar.


    Ok here are the pics of the refurbished 800
    Tom wasnt happy with the power switch/potentiometer so we replaced that with a new one from the gent in NY that sells on EBay.
    He also added a couple tube covers that were missing.

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]
     
  5. Catmanboo

    Catmanboo AK Subscriber Subscriber

    Messages:
    529
    Location:
    Atwater, ohio
    Cool beans! A few things to do in your leisure time: you'll want to reinstall the dashboard light shrouds, those led replacements fire vertically. Thatll make 'em shine in the proper direction ( toward the tuning meter & stereo indicator, which is kind of a dummy meter.) couple longevity mods between the phase inverters & output tubes, cheap & easy, check forums for details. You still have the decals for the tuner & top of chassis, bonus! Also, the big Brown resistor just north of the new bias cap has done some serious cookin'in the past, it may need to go. :D
     
  6. Masonavenut

    Masonavenut AK Subscriber Subscriber

    Messages:
    146
    Location:
    Winnetka, Ca.
    LOL’ing because that was the first thing that got my attention was those nice new lights in my eyeballs.
    I imagine EBay is the place to find the shrouds. This deal doesn’t have ‘em.

    Edit
    Saw what they are. Wonder if they’ll fit on these LEDs.
     
    Last edited: May 24, 2018

     

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  7. Catmanboo

    Catmanboo AK Subscriber Subscriber

    Messages:
    529
    Location:
    Atwater, ohio
    Screw fleabay, many of us likely have a few in the junk bins, I probably do myself, gotta figure out where to start looking. I think they'll fit, those look to be same diameter as stock no.47 bulbs. You could always revert to those & forget the leds, with cabinet on, it'll be ok! Use leds as pilot lites in something else...
     
    Last edited: May 24, 2018
  8. Catmanboo

    Catmanboo AK Subscriber Subscriber

    Messages:
    529
    Location:
    Atwater, ohio
    Here's what you want. Oem shroud on the right, next to proper bulb style. Also shown are red & green covers, looks cool on one of my 500c's, changes the whole mojo of it in the subdued light. I have surplus colored covers, can send ya a set if interested.
     

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  9. Masonavenut

    Masonavenut AK Subscriber Subscriber

    Messages:
    146
    Location:
    Winnetka, Ca.
    Sweet, I’ll look around for those. I have no problem with giving up the LEDs. Those look like bulbs I use in my old cars.
     
  10. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Location:
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    I just take an old tube box (usually for a 12ax7) and cut it down to fit on the top of the meter back, clear the tuning cords, and cut a hole in the side for the lamp. It will aim all the available light in the direction of the meter and limit the overshoot from the lamp even with the shroud on it.
     
  11. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

    Messages:
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    Location:
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    On the 800 if you look at the schematic, the COM (Common or speaker ground) is NOT TIED TO THE CHASSIS. It's grounded thru the 4 ohm speaker tap at the impedance selector. This is somewhat typical of FISHER Receivers and amps of the era, and different from other setups that have the common or Ground terminal to chassis ground, in that if you have a speaker box setup where one set of speakers is connected via the box to two receivers or Amps, you could do damage to one or the other amps or BOTH amps when switching between them. I would think a break before make rotary switch with an unused set of contacts between the two active sets of contacts on the switch would be the best way to go, otherwise extend the receiver spkr wires to the front and terminate them in a Molex type female plug, with the speakers having the Male end. Then just turn off the units, swap the spkr plus from one to the other and turn on the current unit.
     

     

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  12. Masonavenut

    Masonavenut AK Subscriber Subscriber

    Messages:
    146
    Location:
    Winnetka, Ca.
    Thank you Larry.
    I never intended a hot switch between the two amps, merely a means to run two units through one set of speakers. The other unit would be powered down.
    Would that still be a problem?
    Also happy to hear any better way to do it as I’m limited on room for another set of speakers.
     
    Last edited: May 25, 2018
  13. Masonavenut

    Masonavenut AK Subscriber Subscriber

    Messages:
    146
    Location:
    Winnetka, Ca.
    I’m still digesting the ground being tied through the 4 ohm tap and not to the chassis. Why?
    I’ve got some studying to do.
     
  14. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    As I understand it, it gets the spkr grounds above chassis potential and minimizes Ground loops. Dave is the engineer, He knows how to run the train, and what happens when the damned thing goes off the tracks!:p:biggrin:
     
  15. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
    9,775
    Location:
    Ball Ground, GA
    Grounding the 4Ω tap has everything to do with units that offer a powered center channel output, which the big Fisher receivers did.

    There are a number of ways to provide a powered center channel output without actually using an additional amplifier to provide it. Early efforts merely connected the center channel speaker between the two 16Ω terminals of each channel. This type of connection has the center speaker delivering a "difference" signal. That is, the speaker is delivering a signal that is anything representing a difference between the two stereo signals. Therefore, if a stereo source has violins in the left channel, and horns in the right channel, then the center channel speaker will reproduce both of these instruments together, since each instrument in its own stereo channel represents a difference signal between its own channel, and that of the other channel. This works pretty good until you add in (for example) a voice track that is reproduced equally in both stereo channels. Because the voice appears equally in both channels, there is no difference signal generated -- relative to the voices -- and so the voices do not appear in the center channel speaker in this scenario, while the violins and horns do. As you might imagine then, when a stereo amplifier is playing a true mono source with this arrangement, then the center channel speaker goes completely dead, since there is no difference signal generated at all. What is needed then, is a signal that is available from all sources, representing all the sound presented in both channels. Such a signal is known as a "summing" signal. So how do you generate a powered summing signal at the output of a stereo amplifier? That's where the mysterious grounding of the 4Ω tap comes in.

    As a basic fact of OPT design that employs 4-8-16Ω output tap offerings, the 16Ω connection represents the full secondary winding, while because of the way impedance is calculated, the 4Ω tap is actually placed at a tapping point representing one half (50%) of the 16Ω winding. Therefore, as a function of output level, the signal appearing between the Com and 4Ω tap is exactly equal to that appearing between the 4Ω and 16Ω taps. Since the 4Ω tap is the CT of the winding then, the Com - 4Ω - 16Ω leads can be used as a balanced output, so that if the 4Ω tap is grounded, then 1/2 of the signal appears between the Com terminal and ground (the 4Ω tap), and 1/2 between ground and the 16Ω terminal. However, there is one big difference between these two equal signals appearing at the Com and 16Ω taps (relative to ground): They are out of phase with each other. This is the secret that then makes possible a summing powered center channel output.

    If the 4Ω tap is grounded in both channels and the NFB networks adjusted accordingly, then the regular stereo speakers connected between the Com and (typically) 8Ω tap won't know any difference over that of conventional design where the Com terminal is grounded. However, with the new arrangement, if a center channel speaker is now connected to the Com terminal of one channel, and the 16Ω terminal of the other, the two 1/2 level signals provided add together in the center speaker (because they are out of phase) for a full 100% level signal. And, because the signals add (or sum) together, the center channel then produces sound all the time, being a mono signal for both stereo and mono signals. In other words, because the signals taken from each channel for the center channel speaker are always out of phase with each other in this scenario, the center channel speaker always works, regardless of the type of source selected.

    From the standpoint of the center channel speaker, this works very well. However, because the Com terminals of the receiver operate above ground with this arrangement, if the left and right stereo speakers are then connected to an external speaker switch box that employs an internal connection between the Com terminals of the two channels (typically done to simplify the switches needed), then a short has been created between the two channels because now the 4Ω taps are connected together by way of their ground connection, and the Com terminals are now connected together by way of the internal connected between the Com terminals of the external speaker switch box. Therefore, all amplifiers and receivers that provide this type of powered center channel output cannot use the typical external speaker switch box because of the short circuit they almost always create in this scenario.

    One way around this problem does exists, but only when the stereo speakers are 4Ω speakers. In this case, the Com terminal of the speaker switch box can be connected to the 4Ω terminals (ground) of the receiver, and the Hot terminal of the speaker switch box connected to the 16Ω terminals. In this way, the 4Ω stereo channel speakers are simply connected to the "top" half of the 16Ω winding, rather than the "bottom" half. The impedance between the 4Ω and 16Ω terminals is 4Ω, just as it is between the Com and 4Ω terminals. Because the Com terminals of the switch box are connected to the 4Ω terminals of the amplifier/receiver, the internal connection between the Com terminals of the switch box simply reinforces the common ground connection between the 4Ω terminals of the amplifier/receiver, preventing the short that would occur were the Com amplifier/receiver terminals used with the switch box.

    I hope this helps!

    Dave
     
    Last edited: May 26, 2018
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  16. Masonavenut

    Masonavenut AK Subscriber Subscriber

    Messages:
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    Location:
    Winnetka, Ca.
    You sir, are a prince. Thank you so much Dave for taking the time to answer in such comprehensive fashion.
    I almost understand it now. Once I read your answer a few more times I’ll get it set in my li’l pea brain.
    That was an amazing read.
     

     

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  17. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Another page from Dave's little Red Book of Audio Magic. And this one goes in my 3 ring binder! I thought it had to do with speaker grounding in an elementary way. I learned something new today. I'll never be able to explain it the way Dave does.
     

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